The present invention is directed to the fabrication of bipolar transistors for integrated circuits, and particularly directed to the improvement of the emitter efficiency of a bipolar transistor.
In a forward biased bipolar transistor the flow of current from the emitter to the base comprises two components. For example, in an N-P-N transistor, the emitter current consists of forward injected electrons crossing from the emitter into the base, and reverse injected holes crossing from the base into the emitter. It is generally desirable to limit the reverse injection current in a transistor, since the current which results from the holes crossing the emitter junction from base to emitter (in an N-P-N transistor) does not contribute carriers which can eventually reach the collector of the transistor.
In the past, the suppression of the reverse injected carriers has been accomplished by doping the emitter much more heavily than the base region of the transistor. Such a feature insures that the emitter current will consist almost entirely of forward injected carriers. However, such an approach places a limitation on the doping of the base region, and hence limits the frequency response of the transistor.
Another approach to the suppression of reverse injection utilizes variations in the energy gap between the emitter and the base to present a greater barrier to reverse injection than to forward injection. In fact, Shockley generally alluded to such an approach in his basic patent on the transistor, U.S. Pat. No. 2,569,347. However, due to technological limitations, such an approach has not heretofore been employed in any practical, i.e., commercial applications.
Recently, two epitaxial processes, namely molecular beam epitaxy and metal-organic chemical vapor deposition, have been proposed as viable techniques for producing an emitter with a wider energy gap than a base. See the article by Herbert Kroemer entitled "Heterostructure Bipolar Transistors and Integrated Circuits" appearing in Proc. IEEE, Vol. 70, No. 1, January 1982 at pages 13-25. It has yet to be determined whether the proposed techniques are capable of practical applications. For example, when silicon is the material in which the transistor is formed, it is very difficult to grow a thin layer (on the order of 0.1-0.2 microns) of material having a different energy gap and yet maintain the integrity of that layer. In addition, molecular beam epitaxy is a relatively slow, and hence expensive, process. In any event, it appears that the proposed epitaxial techniques are limited to the use of III/V compound semiconductors to form heterostructures.